1. Field of the Invention
The present invention relates to a control apparatus for controlling an electric power steering system that imparts an assist force produced by a motor to the steering system of cars or vehicles. More particularly, the present invention relates to a control apparatus for controlling an electric power steering system improving the feeling of steering of, particularly, when a steering wheel is sharply operated.
2. Description of the Related Art
In an electric power steering system which assists the load of the steering system of a car or a vehicle by utilizing the rotational force of a motor, the driving force of the motor is used for assisting the load of a steering shaft or a rack shaft through reduction gears and a transmission mechanism such as gears or a belt. In order to precisely produce an assist torque(steering assist torque), the conventional electric power steering system controls a motor current by feedback. The feedback control is to so adjust the voltage applied to the motor that a difference decreases between the current control value and the detected motor current. The voltage applied to the motor is usually adjusted relying upon a PWM(pulse width modulation)-controlled duty ratio.
Here, a general constitution of the electric power steering system will be described with reference to FIG. 1. A shaft 2 of a steering wheel 1 is coupled to tie rods 6 of the wheels through reduction gears 3, universal joints 4a and 4b and a pinion-rack mechanism 5. The shaft 2 is provided with a torque sensor 10 for detecting the steering torque. A motor 20 is coupled to the shaft 2 via the reduction gears 3 to assist the steering force of the steering wheel 1. An electric power is supplied from a battery 14, through an ignition key 11 and a relay 13, to a control unit 30 that controls the power steering system. The control unit 30 operates a steering assist command value I for an assist command based on a steering torque T(main torque signal Tm, sub-torque signal Ts) detected by the torque sensor 10, on a vehicle speed V detected by the vehicle speed sensor 12, and controls the electric current supplied to the motor 20 based on the steering assist command value I that is operated.
The above electric power steering system is provided with a torque sensor 10 that detects the steering torque to give, to the motor 20, the steering assist force corresponding to the steering torque transmitted from the steering shaft. To realize the failsafe function, the torque sensor 10 has two input systems to the control unit 30, i.e., a main system and a sub-system, and uses the main torque signal Tm for the steering assist control and uses the sub-torque signal Ts for the failsafe function. Though the steering assist control is carried out based on the main torque signal Tm, the assist control is generally realized by using an MCU (micro controller unit) and, hence, the main analog torque signal Tm must be converted into a digital value through an A/D conversion. However, an A/D converter has a limit on its resolution. In order to maximize the resolution of the A/D converter, therefore, the main torque signal Tm is multiplied (amplified) by a gain times in an analog manner, so that a range of the steering torque T that is usually used becomes a maximum input to the A/D converter. That is, a maximum expected steering torque T is adjusted by being multiplied by the gain times so as to be corresponded to a maximum value of the A/D converter.
FIG. 2 is a block diagram illustrating tie constitution of the control unit 30 of when the signals of the torque sensor 10 are to be controlled through the main and sub two systems. A main torque signal Tm from the torque sensor 10 is inputted to an A/D converter 321 and is further inputted to an A/D converter 322 through an(analog type) gain multiplier(amplifier) 31. A sub-torque signal Ts from the torque sensor 10 is inputted to an A/D converter 323 and an output of which is used for a separate failsafe function.
A main digital torque input value Tgr from the A/D converter 321, too, is used for the failsafe function. A gain-multiplied main torque input signal Tgi from the A/D converter 322 is inputted to a steering assist command operation unit 325 and to a center response improving unit 324, and is used for the failsafe function.
The outputs of the center response improving unit 324 and of the steering assist command operation unit 325 are added up together through an adder 32A, the added result is inputted to a current control unit 201 through a robust stabilization compensation unit 326, an adder 32B and an adder/subtractor 32C, and the motor 20 is driven with a current control value. A motor terminal voltage Vm and a motor current “i” are inputted to a motor angular velocity estimating unit 329, and an estimated angular velocity ω is inputted to a motor inertia compensation unit 328 and to a convergence control unit 327. The output of the convergence control unit 327 is inputted to the adder 32A and is added up, and the output of the motor inertia compensation unit 328 is inputted to the adder 32B and is added up.
The block 30 forms a control unit, and the block 32 forms an MCU control block. That is, the control unit 30 is constituted by the MCU control block 32 and the gain multiplier 31.
The center response improving unit 324 is to enhance the response characteristics of control near the neutral position of steering and to realize a smooth steering. Namely, this is to improve the response characteristics in the assist torque and to improve stability in the torque control system as has been described in, for example, Japanese Patent Application Laid-open No. 2001-328553 A filed by the present applicant. In order to improve the response characteristics in the control system, a value in proportion to the differentiation of the steering torque signal is added to the assist quantity (steering assist command value) while varying the differential gain depending upon the steering torque and the vehicle speed. By continuously changing the differential gain as described above, the steering feeling is prevented from becoming unnatural since there is no great change in the differential gain when the steering torque, vehicle speed or steering pattern has changed and, thus, a comfortable steering performance is obtained. Upon increasing the differential gain in a region where the steering torque is small, further, the response characteristics are enhanced near the neutral point to obtain small hysteresis characteristics, to obtain comfortable steering performance, and to maintain response performance and stability in a region of large steering torque.
In order to improve the convergence of yawing of the vehicle, the convergence control unit 327 applies the brake to the swinging motion of the steering wheel, and the motor inertia compensation unit 328 compensates the inertia and friction of the motor 20. The robust stabilization compensation unit 326 is the one that has been taught in Japanese Patent Application Laid-open No. 8-290778 A, has a characteristic equation G(s)=(s2+a1·s+a2)/(s2+b1·s+b2) using “s” as the Laplace operator, removes a peak value of resonance frequency in the resonance system constituted by an inertia element and a spring element included in the detected torque, and compensates for a deviation in the phase of the resonance frequency that impairs the stability and response characteristics of the control system. Symbols “a1, a2, b1 and b2” in the characteristic equation G(s) respectively denote parameters determined by resonance frequencies in the resonance system.
The motor angular velocity estimating unit 329 may estimate the angular velocity by a method disclosed in Japanese Patent Application Laid-open No. 10-109655 A or may estimate the angular velocity in a manner as taught in Japanese Patent Application No. 10-338152 A filed by the present applicant.
In the above control apparatus, when the steering wheel is sharply steered, the main torque signal Tm from the torque sensor 10 becomes excessively great, and the torque signal Tgo after multiplied by the gain through the gain multiplier 31 may exceed the input range(full-scale) of the A/D converter 322, whereby a value to be put to the A/D conversion is clamped to a maximum value of the A/D converter. When a gain-multiplied torque input value Tgi shown in FIG. 3A is inputted to the center response improving unit 324, there is obtained a waveform shown in FIG. 3B since the center response improving unit 324 is constituted by a differential element as described above. When the gain-multiplied torque input value Tgi is saturated, therefore, the waveform becomes as shown in FIG. 3B. There exists a differential waveform which works to temporarily decrease the current command value as surrounded by a circle and designated at “A”, deteriorating the feeling of steering. The occurrence of this problem will be described below in further detail.
The center response improving unit 324 has gain-frequency characteristics as shown in FIG. 4A and approximate differential characteristics with phase-frequency characteristics as shown in FIG. 4B. The steering assist command operation unit 325, on the other hand, has input/output characteristics of gain-multiplied torque input value (Tgi) vs output as shown in, for example, FIG. 5.
Here, when the steering wheel is sharply steered as described above, the torque sensor 10 produces a large main torque signal Tm and a large sub-torque signal Ts, and the gain-multiplied signal that is multiplied by the gain through the gain multiplier 31 may exceed the full-scale of the A/D converter 322 (time point t1 in FIGS. 6A to 6D). In such a case, the gain-multiplied torque input value Tgi outputted from the A/D converter 322 becomes as shown in FIG. 6A, the output of the steering assist command operation unit 325 becomes as shown in FIG. 6B, and the out of the center response improving unit 324 becomes as shown in FIG. 6C. Further, the output waveform of the “center response improving unit 324+ steering assist command operation unit 325” becomes as shown in FIG. 6D which is the addition of the waveform of FIG. 6B and the waveform of FIG. 6C.
The gain-multiplied torque input value Tgi at the time point t1 is limited to a maximum value of the A/D converter 322, whereby a pulse-like differential waveform appears on the output of the center response improving unit 324 (portion “A” in FIG. 3B and portion “B1” in FIG. 6C) thereby to decrease the output “output of the center response improving unit 324+ output of the steering assist command operation unit 325” (portion “B2” in FIG. 6D). Therefore, the assist torque fluctuates with a large steering input to deteriorate the feeling of steering.
Further, when the gain-multiplied torque input value is received in excess of the resolution of the A/D converter, the steering assist force is not quite controlled for the greater steering torque, often causing offensive feeling such as fluctuation in the steering torque.